1. Field of the Invention
The present invention relates to a photovoltaic device wherein a plurality of unit photovoltaic cells is stacked in series.
2. Description of the Prior Art
As is disclosed in, for example, U.S. Pat. No. 4,271,328, there has been known a photovoltaic device of so-called tandem structure wherein two, three or more of unit photovoltaic cells each having a semiconductive junction such as p-i-n or p-n.sup.- -n.sup.+ are stacked in series. In such a photovoltaic device of tandem structure, the light which has passed a preceding unit photovoltaic cell from the incident side of the photovoltaic device without contributing photovoltaic action can be absorbed in a following unit photovoltaic cell so that the total photovoltaic efficiency can be improved. Further, if the width E.sub.gopt of an optically forbidden band of an optically active layer, such as the i-type layer in a p-i-n junction or the n.sup.- type layer in a p-n.sup.- -n.sup.+ junction of a unit photovoltaic cell, which generates photocarriers mainly upon receiving an incident light is controlled, the peak wavelength in the photosensitivity thereof can be shifted so that the photovoltaic efficiency can be further improved.
Photocarriers (electrons and holes) generated in an optically active layer are attracted by a junction electric-field between the p-type and the n-type layers which sandwich the optically active layer. Namely, electrons are moved toward the n-type layer while holes are moved toward the p-type layer, and then they are collected and outputted. Therefore, in a unit photovoltaic cell, not only an undoped or lightly doped optically active layer such as an i-type layer or n.sup.- -type layer contributes to the generation of electric power, but also impurity doped layers for generating a junction electric-field are required.
However, because each optically active layer is formed to be interposed in the path of incident light between impurity doped layers which are necessary for generating a junction electric-field, the amount of light reaching the optically active layer decreases if the absorption of light increases in the impurity doped layers and, therefore, this causes a serious decrease in the photovoltaic efficiency.
In a photovoltaic device disclosed in U.S. Pat. Nos. 4,385,199 or 4,388,482 comprising one unit photovoltaic cell and impurity layers, a so-called window layer arranged in front of an optically active layer at the light-impinging side is made of a wide-band gap material such as amorphous silicon carbide or amorphous silicon nitride which has a width E.sub.gopt of the optically forbidden band wider than that of the optically active layer in order to decrease the absorption of light in the window layer.
Therefore, if such a wide-band gap material effective for decreasing the absorption of light as mentioned above is used for the impurity doped layers required for generating a junction electric field which themselves scarcely contribute to the generation of electric power in a photovoltaic device of tandem structure, the absorption of light in the impurity layers can be decreased and, accordingly, the photovoltaic efficiency can be improved.
On the other hand, it has been known that the photovoltaic efficiency of a photovoltaic device made with amorphous silicon decreases when subjected to strong light irradiation for a long time. Even if the initial photovoltaic efficiency thereof can be improved by using a wide-band-gap material as mentioned above, the increase in the photovoltaic efficiency may be canceled when the temporal change thereof is serious and, further, it may happen that after a long period of time the photovoltaic efficiency becomes lower than that of a photovoltaic device of a prior-art structure. Therefore, researches for improving the photovoltaic efficiency and decreasing the temporal change have been conducted in parallel in recent years.